1. Field of The Invention
The present invention generally relates to a method for controlling the localization (hereunder sometimes referred to as sound image localization) of a sound source image (incidentally, a sound source image is a listener's acoustic and subjective image of a sound source and will hereunder be referred to simply as a sound image) in such a manner to be able to make a listener feel that he hears sounds emitted from a virtual sound source (namely, the sound image) which is localized or located at a desired position being different from the position of a transducer (for example, a speaker), and more particularly to a method for controlling the localization of a sound image, which can be employed by what is called an amusement game machine (namely, a computer game (or video game) device) and a computer terminal and can reduce the size of a circuit without hurting the above-mentioned listener's feeling about the sound image localization. Further, the present invention relates to a method for reproducing sounds from signals, which are supplied from a same sound source through a plurality of signal conversion circuits, by using transducers disposed apart from each other and for controlling the localization of a sound image in such a way to be able to make a listener feel that he hears sounds from a virtual sound source (namely, the sound image) which is localized at a desired position being different from the positions of the transducers (for instance, speakers). Especially, the present invention relates to the improvement of calculation of data to be used for controlling the sound image localization (namely, the improvement of calculation of transfer characteristics of signal conversion circuits).
2. Description of The Related Art
A conventional sound image localization method employs what is called a binaural technique which utilizes the signal level difference and phase difference (namely, time difference) of a same sound signal issued from a sound source between the ears of a listener and makes the listener feel as if the sound source were localized at a specific position (or in a specific direction) which is different from the actual position of the sound source (or the actual direction in which the sound source is placed).
A conventional sound image localization method utilizing an analog circuit, which was developed by the Applicant of the instant application, is disclosed in, for example, the Japanese Laying-open Patent Application Publication Official Gazette (Tokkyo Kokai Koho) NO. S53-140001 (namely, the Japanese Patent Publication Official Gazette (Tokkyo Kokoku Koho) NO. S58-3638)). This conventional method is adapted to enhance and attenuate the levels of signal components of a specific frequency band (namely, controls the amplitude of the signal) by using an analog filter such that a listener can feel the presence of a sound source in front or in the rear. Further, this conventional method employs analog delay elements to cause the difference in time or phase between sound waves respectively coming from the left and right speakers (namely, controls the phase of the signal) such that a listener can feel the presence of the sound source at the left or right side of him.
However, this conventional sound image localization method employing an analog circuit as described above has drawbacks in that it is very costly and difficult from a technical point of view to precisely realize head related characteristics (namely, a head related transfer function (hereunder abbreviated as HRTF)) in connection with the phase and amplitude corresponding to each frequency of the signal and that generally, it is very difficult to localize the sound source at a given position in a large space which subtends a visual angle (namely, the difference between maximum and minimum azimuth angles measured from the listener's position) of more than 180 degrees at the listener's eye.
Further, there has been another conventional sound image localization method realized with the recent progress of digital processing techniques, which is disclosed in, for instance, the Japanese Laying-open Patent Application Publication Official Gazette NO. H2-298200 (incidentally, the title of the invention is "IMAGE SOUND FORMING METHOD AND SYSTEM").
In case of this sound image localization method using a digital circuit, a Fast Fourier Transform (FFT) is first performed on a signal issued from a sound source to effect what is called a frequency-base (or frequency-dependent-basis) processing (i.e., a processing to be performed in a frequency domain (hereunder sometimes referred to simply as a frequency-domain processing)), namely, to give signal level difference and a phase difference, which depend on the frequencies of signals, to left and right channel signals. Thus, the digital control of sound image localization is achieved. In case of this conventional method, the signal level difference and the phase difference at a position at which each sound image is located, which differences depend on the frequencies of signals, are collected as experimental data by utilizing actual listeners.
Such a sound image localization method using a digital circuit, however, has drawbacks in that the size of the circuit becomes extremely large when the sound image localization is achieved precisely and accurately. Therefore, such a sound image localization method is employed only in a recording system for special business use. In such a system, a sound image localization processing (for example, the shifting of an image position of a sound of an air plane) is effected at a recording stage and then sound signals (for instance, signals representing music) obtained as the result of the processing are recorded. Thereafter, the effects of shifting of a sound image is obtained by reproducing the processed signal by use of an ordinary stereophonic reproducing apparatus.
Meanwhile, there have recently appeared what is called an amusement game machine and a computer terminal, which utilize virtual reality. Further, such a machine or terminal has come to require real sound image localization suited to a scene displayed on the screen of a display thereof.
For example, in case of a computer game machine, it has become necessary to effect a shifting of the sound image of a sound of an air plane, which is suited to the movement of the air plane displayed on the screen. In this case, if the course of the air plane is predetermined, sounds (or music) obtained as the result of shifting the sound image of the sound of the air plane in such a manner to be suited to the movement of the air plane are recorded preliminarily. Thereafter, the game machine reproduces the recorded sounds (or music) simply and easily.
However, in case of such a game machine computer terminal), the course (or position) of an air plane changes according to manipulations performed by an operator thereof. Thus, it has become necessary to perform a real-time shifting of a sound image according to manipulations effected by the operator in such a way to be suited to the manipulations and thereafter reproduce sounds recorded as the result of the shifting of the sound image. Such a processing is largely different in this respect from the above described sound image localization for recording.
Therefore, each game machine should be provided with a sound image localization device. However, in case of the above described conventional method, it is necessary to perform an FFT on signals emitted from a sound source and the frequency-base processing (namely, the frequency-domain processing) and to effect an inverse FFT for reproducing the signals. As a result, the size of a circuit used by this conventional method becomes very large. Consequently, this conventional method cannot be a practical measure for solving the problem. Further, in case of the above described conventional method, the sound image localization is based on frequency-base data (or data in a frequency domain (namely, data representing the signal level difference and the phase difference which depend on the frequency of a signal)). Thus, the above described conventional method has a drawback in that when an approximation processing is performed to reduce the size of the circuit, transfer characteristics (or an HRTF) cannot be accurately approximated and thus it is difficult to localize a sound image in a large space as subtending a visual angle of more than 180 degrees at a listener's eye. The present invention is accomplished to eliminate such a drawback of the conventional method.
It is, accordingly, an object of the present invention to provide a method for controlling sound image localization, which can reduce the size of a circuit to be used and the cost and can localize a sound image in a large space as subtending a visual angle of more than 180 degrees at a listener's eye. As will be described later, an aspect of such a method resides in that a sound image is localized by processing signals issued from a sound source on a time base or axis (namely, in a time domain) by use of a pair of convolvers. Thereby, the size of the circuit can be very small. Further, this method can be employed in a game machine for private or business use. Moreover, another aspect of such a method resides in that data for a sound image localization processing by the convolvers is finally supplied as data for a time-base impulse response (namely, an impulse response obtained in a time domain (hereunder sometimes referred to simply as a time-domain impulse response)). Thereby, transfer characteristics can be accurately approximated without deteriorating the sound image localization and the size of a circuit (thus, the number of coefficients of the convolvers) can be further smaller.
In case of this new method for controlling sound image localization, the time response (namely, transfer characteristics) of the convolver is obtained from results of the measurement of HRTF. However, if the characteristics are considered as the frequency response, the characteristics have sharp peaks and dips.
In case where such transfer characteristics (namely, the time response) are used as those of the convolver without any modification, sound quality obtained at the time of implementing sound image localization becomes unnatural due to the presence of the peaks and dips in the frequency characteristics. This means that there is some limit to the actual measurement of HRTF.
Moreover, the time response (namely, the impulse response) per se also has sharp peaks and dips. This results in that the convergency of the convolver is not sufficient and thus the size of the circuit (namely, the number of coefficients of the convolver) does not become so small. The present invention further seeks to solve such problems.
It is, therefore, another object of the present invention to provide an improved method for controlling sound image localization, which can improve the calculation of transfer characteristics of a signal conversion circuit and also improve the sound quality and reduce the size of the circuit.